This invention relates to an improved catalyst active in ammonia production from ammonia synthesis gas. Furthermore, it relates to the recovery of useful catalytic components from a spent catalyst.
During the 20th century, the production of ammonia has increased dramatically to put ammonia among the most important industrial chemicals in terms of tonnage. Today, more than 1% of the global energy consumption is used for manufacture of ammonia in the classical Haber-Bosch process. Currently, the multi-promoted iron catalysts are by far the most important catalysts used industrially.
Recently, a promoted ruthenium (Ru) catalyst supported on carbon has been introduced into commercial operation. A general drawback of the ruthenium catalyst is the carbon support when used under industrial conditions, since all forms of carbon are susceptible to hydrogenation under these conditions. Hydrogenation results in a gradual loss of the carbon support by methane formation and eventually into severe operating difficulties. Consequently, support materials that are thermodynamically stable under the industrial ammonia synthesis conditions, i.e. at temperature between 300xc2x0 C. and 500xc2x0 C. and pressures above 50 bar (50xc3x97105 kPa), can be expected to be important alternatives to the carbon support.
U.S. Pat. No. 4,309,311, which is incorporated herein by reference, discloses a catalyst for ammonia synthesis obtained by heating a composition of a cyano complex, at a temperature of approximately 325-430xc2x0 C. This catalyst is based on an alkali metal or alkaline earth metal hexacyano-ruthenate on a support of, for example, magnesia. The activated form of this catalyst is the salt of the cyanocomplex, and the nature of the support is not stated as having any particular significance.
U.S. Pat. No. 4,588,705 discloses a gel or suspension for multiple purposes, amongst others ammonia production, which is based on a mixture of a cyano-containing anionic complex having a reducible transition metal and a non-reducible metal, with one or more solutions containing polyvalent metal ions.
U.S. Pat. No. 4,698,325 discloses a catalyst for the preparation of ammonia. Examples of catalyst precursors are shown, the precursors being based on a fused mixture of oxides containing 0.3% w/w magnesia and using a promoter of an alkali metal salt of ruthenium.
Various ruthenium-containing catalysts for use in other processes such as production of synthesis gas are mentioned in, for instance, U.S. Pat. Nos. 5,756,421 and 5,336,655, and SE 507,226.
The patent disclosures mentioned above are all incorporated herein by reference.
Since ruthenium-based ammonia synthesis catalysts are much more expensive than iron-based catalysts it is essential to ensure optimal usage of the ruthenium in the catalyst. Therefore, the accessible surface area of such a ruthenium catalyst must be suitable to give a high catalytic activity and a long operation time of the catalyst. Furthermore, it is essential that ruthenium can be recovered from the catalyst at a low cost to enable recycling of the ruthenium metal.
Lately, we have investigated the use of a support material, which allows the preparation of a catalyst with a high dispersion of Ru and easy recovery of both Ru as well as the necessary promoters. We have found that in this respect (and with respect to susceptibility to hydrogenation as previously mentioned) magnesium oxide (MgO) is a very desirable support. Previously, MgO supported promoted Ru catalysts active in ammonia synthesis have been reported in the literature.
We have found that a MgO supported promoted Ru catalyst exhibits unexpectedly much higher catalytic activity in ammonia production from ammonia synthesis gas compared to those reported earlier, when the catalyst support has a specific area of above 40 m2/g and the ruthenium content of the catalyst is between 3 wt % and 20 wt % and the promoter content is between 0.2-0.5 mole of ruthenium. Furthermore, a process was found to be suitable for the recovery of useful catalytic components, especially for the recovery of ruthenium from spent catalyst.
Accordingly, it is an object of the invention to provide a catalyst for the synthesis of ammonia.
It is another object of the invention to provide an ammonia synthesis catalyst exhibiting higher catalytic activity than those previously known.
Furthermore, it is an object of the invention to provide a catalyst containing catalytic compounds which can easily be recovered from the spent catalyst.
Finally, it is an object of the invention to provide a process for the recovery of catalytic components from the ammonia synthesis catalyst.
It is the general perception that there is a proportionality relation between the activity (moles of ammonia produced/g of catalyst) and the catalyst content of ruthenium, in the group of catalysts in question. By the present invention the activity of the catalyst was increased by a factor 10-20, while the Ru content was increased by a factor 1.5-8.
Thus, the invention provides a catalyst active in ammonia synthesis with improved activity. The invention also provides a process for the recovery of useful catalytic components from the catalyst, i.e.
I) a solution of dissolved hydroxides of the promoter components,
II) a solution of a dissolved magnesium compound, and
III) ruthenium metal.
It was further found that the activity of a catalyst according to the invention is improved at operating conditions, where the ammonia concentration is high and/or where the composition of the synthesis gas is low in hydrogen compared to stoichiometry of the ammonia formation reaction scheme.
In industrial applications this increased activity has several advantages. Significantly improved catalyst activity allows use of a correspondingly smaller synthesis converter or alternatively use of a lower synthesis pressure without lowering the ammonia production rate.